In processing plants, burners are used to heat process fluids and to burn off waste gas. Such burners generate one or more flames within a firebox using a fuel gas and a source of oxygen such as air. Changes in the quantity or quality of the fuel gas, such as changes in the BTU content of the fuel gas, or changes in the availability of oxygen in the firebox can lead to instability of the burner flames. If the proper mixture of fuel and oxygen is not maintained, the burner may move into a phase of sub-stoichiometric combustion, in which there is not enough oxygen present for full combustion of the fuel. Sub-stoichiometric combustion may result in burner flames going unstable and if not immediately corrected, the burner flames may be completely extinguished. This is referred to as flame-out.
It is important that flame-outs be detected quickly because after a flame is extinguished, uncombusted fuel fills the firebox. If this fuel is ignited, it is possible for it to explode, thereby damaging the burner/heater.
One measure of flame performance is firebox draft, which can be measured by measuring the pressure difference between the pressure within the firebox and the pressure outside the firebox. This pressure difference is referred to as a draft pressure or draft measurement. Recently, statistical process monitoring technology has been applied to firebox draft measurements. In statistical process monitoring, the pressure values for the firebox draft are statistically processed to determine a mean and standard deviation of the pressure samples. The standard deviation of the draft pressures is then used to detect flame instability.